Calculate vig, overround and no-vig probabilities
Vig, overround and bookmaker margin describe how far a market's combined implied probabilities sit above 100%. Normalisation removes that excess proportionally; it does not prove the resulting probabilities are correct.
Formula and worked example
Book percentage = Sum of (1 / Decimal odds)
Overround = Book percentage - 100%
No-vig probability = Outcome implied probability / Book percentage
A two-way market at 1.80 and 2.10 implies 55.56% and 47.62%. The book percentage is 103.17%, so the overround is 3.17%. Normalisation produces no-vig probabilities of about 53.85% and 46.15%.
Reading the result
Book percentage measures the full set of entered outcomes. Missing an outcome makes the result incomplete. A negative overround can occur when prices from different sources create an arbitrage rather than a conventional single-book market.
Limits and responsible use
The no-vig split is a proportional estimate. It does not model information, bias or execution risk. Settlement and void rules can also change the realised result. See Gambling Commission LCCP 4.2.6.
Set a spending limit before gambling and do not chase losses. Read GambleAware's advice and our responsible gambling guidance.
Related tools and guides
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Sources
- Gambling Commission, LCCP condition 4.2.6. Current code accessed 30 July 2026.
- GambleAware, Advice to consider if you're gambling. Accessed 30 July 2026.